1. Technical Field
The present disclosure relates in general to a circuit for power management of a standard cell application. In particular, the present disclosure relates to a circuit for power management of a static random access memory (SRAM).
2. Description of the Related Art
Many integrated circuit devices, such as microprocessors, include on-board memory devices, such as SRAM devices. For example, SRAM devices are commonly used as cache memory because of their relatively fast speed. SRAM devices are also sold as stand-alone integrated circuits for use as cache memory and for other uses. SRAM devices are also more suitable for use as cache memory than dynamic random access memory (“DRAM”) devices because they need not be refreshed, thus making all SRAM memory cells continuously available for a memory access.
FIG. 1 is a block diagram of a portion of a typical array 10 of SRAM cells 12 arranged in rows and columns. A plurality of complementary digit line pairs D, D are used to couple complementary data to and from the memory cells 12 in a respective column. Several digit line pairs, typically 16 or 32 digit line pairs, are coupled to respective inputs of a column multiplexer 13. The column multiplexer 13 couples one pair of digit lines corresponding to a column address to a sense amplifier 14 and a write driver 16. The sense amplifier 14 provides a data output (not shown) indicative of the polarity of one digit line D relative to the other D responsive to data being read from a memory cell 12 coupled to the selected digit line pair D, D. The write driver 16 drives a differential voltage onto the digit lines D, D to which the write driver 16 is coupled by the column multiplexer 13. The differential voltage applied between the digit lines is indicative of data that is to be written to a memory cell 12 coupled to the digit lines D, D. An equilibration PMOS transistor 18 is also coupled between each pair of complementary digit lines D, D to equalize the voltage between the digit lines D, D prior to a memory read operation. Finally, a complementary PMOS bias transistor 20 is coupled to each digit line D, D to lightly bias the digit lines D, D to VCC. The current provided by each pair of bias transistors is controlled by a respective digit line load signal DLLN.
A plurality of word lines WLI–WL4 activates the memory cells 12 in the respective row of memory cells. The word lines WL1–WL4 are coupled to a respective inverter 22 each formed by a PMOS transistor 24 and an NMOS transistor 26 coupled in series between VCC and ground. The gates of the transistors 24, 26 are coupled to each other and to a respective select line SEL WL1–SEL WL4.
FIG. 2 shows a conventional 6-transistor (6-T) SRAM cell. The SRAM cell 12 includes a pair of NMOS access transistors 122 and 124 that allow a differential voltage on the digit lines D, D, to be read from and written to a storage circuit 30 of the SRAM cell 12. The storage circuit 30 includes NMOS pull-down transistors 32 and 36 that are coupled in a positive-feedback configuration with PMOS pull-up transistors 34 and 38, respectively. Nodes A and B are complementary input/output nodes of the storage circuit 30, and the respective complementary logic values at these nodes represent the state of the SRAM cell 12. For example, when the node A is at logic “1” and the node B is at logic “0”, the SRAM cell 40 is storing a logic “1”. Conversely, when the node A is at logic “0” and the node B is at logic “1”, the SRAM cell 12 is storing a logic “0”. Thus, the SRAM cell 12 is bitable, i.e., the SRAM cell 12 can have one of two stable states, logic “1” or logic “0”.
However, the conventional circuits described suffer standby leakage problems when the circuits are in standby mode. Standby leakage problems are serious concerns in very deep submicron technology with device size reductions, causing output state of memory cells changed. FIG. 3 shows current leakage sources in a transistor 40. The transistor 40 comprises a gate 42, a source 44, a drain 46 and a well 48. Current leakage is caused by junction leakage I1, weak inversion I2, drain induced barrier lowering I3, gate induced drain leakage I4, punchthrough I5, narrow width effect I6, gate oxide tunneling I7 and hot carrier injection I8.
The increased subthreshold leakage and gate leakage current not only increase the IC reliability issues, but also increase the package cost in order to handle the excess power dissipation. The rapidly increased leakage current leads the huge power consumption when the IC chip is getting larger, faster and denser. Thus, power management techniques become a required design issue. Recently, a patent disclosed a method of handling excessive power consumption problems. Some design issues and limitations, however, were not addressed by the patent.
U.S. Pat. No. 6,664,608 to Burr, et al. discloses a back-biased MOS device. Both p-wells and n-wells are formed on a front side of a bulk material. The N devices and P devices are formed respectively within the P-wells or N-wells. The P-wells or N-wells are electrically isolated from one another and are routed to different potentials to vary their threshold voltages. The changed threshold voltages are than used to reduce subthreshold leakage current thereof.
The leakage of the PMOS transistor is reduced by the back-biased MOS method. The NMOS transistor leakage issue, however, still exist. In addition, the back-biased MOS method presents junction breakdown and gate oxide breakdown concerns and offers limited power savings in very deep submicron technology. Very deep submicron technology has less efficient threshold voltage variation due to use of the backed-gate bias, thus it cannot solve the gate leakage problem for 90 nm technology and beyond.
Leakage of a memory cell is reduced by powering down all the memory cells or reducing the power supply voltage. However, powering down all the memory cells results in the loses of the data stored therein, and a power regular is required to reduce the power supply voltage, thus increasing the design difficulty.